1. Field of the Invention
The present invention relates to a continuous process for filtration of a suspension or a colloidal solution through a filtering device.
The invention also relates to an apparatus for working this process and to control means for said filtering device.
2. Description of the Prior Art
The production of vaccines and other solid or soluble products from microbes or animal cells requires the purification of large amounts of microbes, e.g. bacteria and virus, or animal cells, e.g. tumor cells, and/or the preparation of particle-free growth medium. Presently, for the concentration and washing of cultured particles, which are usually present in a broad concentration range around 1 volume percent, centrifuges are used virtually exclusively. This is true also for the preparation of soluble substances from growth media with a high particle density (solid content). Standard centrifuges with angle rotors or swing-out rotors have obvious drawbacks when processing large volumes because they have to be stopped repeatedly so that the supernate can be manually emptied and it can be filled up again with more suspension. Besides, the recovered particles must be resuspended and washed which is cumbersome since the material is distributed into a number of vessels. Certain centrifuges of specific construction allow continuous filling and discharge of supernate during rotation whereby the particle mass separated in the centrifuge rotor is either recovered as a sediment in the rotor or leaves the rotor continuously as a concentrate (according to the separator principle). Aside from their high price and complicated management, these special centrifuges have the important disadvantage in common with conventional centrifuges that the risks of disseminating dangerous microbes, e.g. through splashing or aerosol formation, are difficult or impossible to eliminate.
Finally one can point to the corresponding difficulties involved in keeping bacteria being washed with centrifuge techniques free from contaminating microbes. This may potentially be of great importance when producing concentrated suspensions of bacteria of so-called starter cultures for the dairy industy. Furthermore, centrifuges do not produce a fluid phase, sufficiently free from solid phase as required for a number of applications.
The last few years' development in filtering technology has resulted, among other things, in the construction of devices with large filtering areas for the concentration and/or dialysis of soluble substances and also for the sterile filtration of solutions of heat labile substances with a minimal content of microbes. The possibility of using conventional filtering devices in order to concentrate and wash, e.g. bacteria cultured in a fluid growth medium is limited by the fact that the filter membranes rapidly will be obstructed or clogged by a film of bacteria. However, it has been demonstrated--in the case of particle separation as well as in the case of so-called ultrafiltration (molecular sieving)--that devices constructed for tangential flow of the fluid under treatment across one surface of the filter membrane, here called the primary surface, allow that before obstruction a (much) larger volume of filtrate can be recovered on the other surface, here called the secondary surface, than in conventional constructions. When using special devices for ultrafiltration comprising bundles of parallel thin tubes, through which the solution or suspension being treated passes according to said principle of tangential flow, one has exploited the possibility, after the use of the filtration device, to regenerate, that is to wash the primary surface free from obstructing colloidal material by arranging a flow of rinsing fluid, which runs through the filter membranes in a direction opposite to that of the filtrate, i.e. from the secondary to the primary surface. A significant drawback of such equipment--exempting the fact that membrane tubes suitable for the filtration of so-called true particle suspensions are not available--is that the "dead volume" is large particularly at the secondary surface of the outside of the thin tubes. This means that, if it is necessary, during filtration, to make an interruption in order to regenerate the filtering device in the manner just mentioned, this will result in a larger quantity filtrate penetrating from the primary surface at the beginning of each rinsing period and leaving the filter together with the solid or colloidal phase from which the liquid phase should be separated. Moreover, the filtrate will be diluted with a considerable amount of rinsing liquid at the beginning of a following filtering period. The same drawbacks prevail in case it is preferred to have the liquid to be filtered outside the parallel thin tubes.